2261 - Assessing Intra-fractional Organ Motion on Prostate Adaptive Radiotherapy

O. Dona Lemus, Y. Zhou, S. Tanny, J. Wancura, M. Webster, J. Yoon, H. Jung, F. Li, A. Podgorsak, M. A. Cummings, and D. Zheng; University of Rochester, Rochester, NY

Purpose/Objective(s): Prolonged on-couch treatment sessions in online adaptive radiotherapy (oART) significantly increase the likelihood of intra-fractional organ motion. In this study, we explore the effects of such changes in the radiation dose delivered to the target volume and adjacent healthy organs in prostate patients undergoing this novel treatment approach. Materials/

Methods: Our cohort consists of 10 patients receiving pelvic oART. Patients received an initial CBCT for oART planning and a positioning verification CBCT following approval of the oART plan, resulting in a dataset of 150 CBCTs. Clinical target volumes, rectum, and bladder structures were assessed as part of this study. Segmentation of the tumor volume and critical organs was conducted on both images and the differences in organ volumes and organ doses were calculated. DVH changes between the two-time points (?t) were assessed using the DVH Gamma criteria which evaluate each DVH point using a volume difference criterion of 3% and a dose difference criterion of 3%. DVH Gamma shows the percent of DVH points that fall within such criteria.

Results: Table 1 shows preliminary findings for one of the evaluated patients. The average time between scans was 18mins (SD 2min) for phase 1 and 11mins (SD 2min) for phase 2. For the pelvis nodes, a passing rate of 100% was recorded for all fractions. For prostate and seminal vesicles, passing rates varied from 54% - 100% (mean 83%, SD 11%). The wide range in volume of bladder (3cc-200cc) and rectum (1cc-54cc) affected the DVH Gamma passing rates, which ranged from 58%-100% (mean 88%, SD 14%) and 20%-100% (mean 53%, SD 24%) respectively.

Conclusion: This study demonstrated substantial changes in organ volumes during oART in the abdominopelvic region. The clinical implications of these changes require further assessment. It is essential to expedite the adaptive plan generation process to unlock the full potential of daily adaptation for targets situated within the abdominopelvic area as our ability to reduce the treatment margins could be unknowingly compromised by the effects of the intra-fractional variability. Abstract 2261 - Table 1: Summary of DVH Gamma Metrics of an Adaptive Patient over 37 FXs

Gamma (3%,3%) Statistics	PTV_PelvicNodes	PTV_Prostate+SV	Bladder	Rectum
Maximum (%)	100.0	100.0	100.0	100.0
90th percentile (%)	100.0	97.3	100.0	87.3
Mean (%)	100.0	83.2	88.4	52.7
50th percentile (%)	100.0	84.6	94.1	47.1
Minimum (%)	100.0	53.8	57.9	20.3